/Engine/Source/Runtime/InteractiveToolsFramework/Public/BaseGizmos/GizmoMath.h
Find the closest point to QueryPoint that is on the circle defined by CircleOrigin, CircleNormal, and CircleRadius.
namespace GizmoMath { void GizmoMath::ClosetPointOnCircle ( const FVector & QueryPoint, const FVector & CircleOrigin, const FVector & CircleNormal, float CircleRadius, FVector & ClosestPointOut ) }